A part of speech used to describe lazy behavior, such as:

"What did you do today?"

"Nothing. I sat around on my ass, ate chips and played Pac-Man."

Most commonly used in an insulting manner describing the behavior of others:

"Why don't you stop sitting around on your ass and help me fix this goddamn stereo you lazy piece of shit".

I find this phrase particularly interesting since anyone who is sitting is, by default, according to the structure of the human body, sitting on their ass, assuming that "ass" describes the mass of muscle and body fat situated on the upper dorsal area of the hips and not Juan Valdez's trusty sidekick.

Maybe industrious people are spending all their time sitting on their hands?